Commit a14602fc authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by Jakub Kicinski
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veth: reduce XDP no_direct return section to fix race 



As explain in commit fa349e39 ("veth: Fix race with AF_XDP exposing
old or uninitialized descriptors") for veth there is a chance after
napi_complete_done() that another CPU can manage start another NAPI
instance running veth_pool(). For NAPI this is correctly handled as the
napi_schedule_prep() check will prevent multiple instances from getting
scheduled, but for the remaining code in veth_pool() this can run
concurrent with the newly started NAPI instance.

The problem/race is that xdp_clear_return_frame_no_direct() isn't
designed to be nested.

Prior to commit 401cb7da ("net: Reference bpf_redirect_info via
task_struct on PREEMPT_RT.") the temporary BPF net context
bpf_redirect_info was stored per CPU, where this wasn't an issue. Since
this commit the BPF context is stored in 'current' task_struct. When
running veth in threaded-NAPI mode, then the kthread becomes the storage
area. Now a race exists between two concurrent veth_pool() function calls
one exiting NAPI and one running new NAPI, both using the same BPF net
context.

Race is when another CPU gets within the xdp_set_return_frame_no_direct()
section before exiting veth_pool() calls the clear-function
xdp_clear_return_frame_no_direct().

Fixes: 401cb7da ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Signed-off-by: default avatarJesper Dangaard Brouer <hawk@kernel.org>
Link: https://patch.msgid.link/176356963888.337072.4805242001928705046.stgit@firesoul


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 4b4749b7

drivers/net/veth.c

+3 −4
Original line number Diff line number Diff line
@@ -975,6 +975,9 @@ static int veth_poll(struct napi_struct *napi, int budget) 


	if (stats.xdp_redirect > 0)

		xdp_do_flush();

	if (stats.xdp_tx > 0)

		veth_xdp_flush(rq, &bq);

	xdp_clear_return_frame_no_direct();



	if (done < budget && napi_complete_done(napi, done)) {

		/* Write rx_notify_masked before reading ptr_ring */
@@ -987,10 +990,6 @@ static int veth_poll(struct napi_struct *napi, int budget) 
		}

	}



	if (stats.xdp_tx > 0)

		veth_xdp_flush(rq, &bq);

	xdp_clear_return_frame_no_direct();



	/* Release backpressure per NAPI poll */

	smp_rmb(); /* Paired with netif_tx_stop_queue set_bit */

	if (peer_txq && netif_tx_queue_stopped(peer_txq)) {